The present invention relates generally to open-center control valves commonly utilized in power steering systems and, more particularly, to such open-center control valves having diversionary slots positioned hydraulically in parallel with their output slots.
In power steering systems, it is desirable to provide the control valve with a static pressure-effort characteristic having desirable on-center effort combined with relatively rapid boost recovery at moderate valve deflection angles. Reaction control valves featuring on-center detent mechanisms are known to provide such characteristics. However, they are typically deemed to be too expensive for use in high volume applications. Accordingly, a first preferred embodiment of the present invention is directed to providing a control valve wherein secondary portions of the hydraulic fluid flowing therethrough are selectively diverted by flow selective diversionary control orifices to diversionary slots in order to provide the above noted characteristics. Generally, the diverted secondary portion of hydraulic fluid decreases in value with increasing values of valve deflection angle. As a side benefit this also provides a more stable pattern of dynamic pressure-effort characteristics whereby significantly increased values of steering effort are required to effect positive values of steering motion.
The use of diversionary slots is known in the art. Generally, they are differently configured and are utilized for other purposes. By way of example, diversionary slots are utilized in conjunction with oversized control orifices in implementing the high speed features of the power steering valves described in SAE Paper 880707 entitled 1988 "LINCOLN CONTINENTAL VARIABLE-ASSIST POWER STEERING SYSTEM" and U.S. Pat. No. 4,651,551 entitled "MEANS FOR MANUFACTURING COMPONENTS OF ROTARY VALVES", issued to Bishop et al. on Mar. 24, 1987. In the power steering valves described in SAE Paper 880707 and U.S. Pat. No. 4,651,551, the diversionary slots become overwhelmingly flow dominant whenever a parallel supplemental flow circuit comprising them is activated at high vehicular speeds. While this enables the speed sensitive steering features of the power steering valve described in SAE Paper 880707 and U.S. Pat. No. 4,651,551, it also severely reduces available fluid flow to that valve's output slots and precludes a normal range of steering motion at low values of applied steering effort. For this reason the supplemental flow circuit can not be fully activated until the host vehicle attains very high speeds (i.e., in excess of 60 mph) where a full range of steering motions is not normally required.
While this limitation is significantly ameliorated in the control valve of the first preferred embodiment of the present invention because of its significantly reduced fraction of diverted hydraulic fluid, there is some residual reduction in available load flow at low values of applied steering effort. Accordingly, in another preferred embodiment of the present invention small transverse slots known as parasitic slots are positioned in parallel with selected ones of the control valve's input or return flow distribution slots. The parasitic slots overlap the control valve's output slots thus forming parasitic flow orifices which provide a selectively variable "leak" across the output slots, and therefore, the host steering system's power cylinder. Putting the "leak" across the power cylinder provides additional flow between left and right portions of the power cylinder at low values of applied steering effort which restores the full range of steering motion and results in enhanced steering return characteristics. Additional parasitic slots can be positioned in parallel with other selected ones of the control valve's input or return flow distribution slots to form additional parasitic flow orifices interconnecting the diversionary slots. This enables further tailoring of the diversionary function in order to maintain sustained levels of apparent system damping.
Other features, objects and advantages of the present invention will become readily apparent to one skilled in the art upon analysis of the following written description taken in conjunction with the accompanying drawings and appended claims.